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Topic 5E Reactions of alkenes and alkynes

Topic 5E Reactions of alkenes and alkynes. 52. Reaction Mechanisms. In an organic reaction: we break bonds and form bonds, and these bonds are covalent; electron pairs are involved

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Topic 5E Reactions of alkenes and alkynes

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  1. Topic 5EReactions of alkenes and alkynes

  2. 52 Reaction Mechanisms In an organic reaction: • we break bonds and form bonds, and • these bonds are covalent; electron pairs are involved • A mechanism describes the sequence in which bond breaking and bond formation occurs as well as how the energy of the system changes during a process • Use arrow notation to depict movement of electrons and energy profile diagrams to depict energy changes during that process.

  3. 53 Reaction Energy Profiles Exothermic reaction pathway:

  4. 53 Reaction Energy Profiles Endothermic reaction pathway:

  5. 54 A two-step reaction • Intermediates lie in shallow energy wells • Rate-determining step— step with highest EA

  6. CH 2 H X RCH CH 2 + H /H OH HO H CH 2 H X 2 2 RCH CH RCH CH 2 2 X X H H 55 Addition reactions summary RCH X H RCH

  7. CH 2 H X CH 2 37 Addition reactions summary RCH X H RCH

  8. CH CH CH CH 2 2 3 2 55 Addition of Hydrogen Halides • Hydrogen halides react with alkenes • Alkyl halides are formed H Br Br Ethyl bromide

  9. CH –CH=CH + H Cl 3 2 CH CHC H CH C H CH Cl 3 3 3 2 2 Not formed Cl 56 Addition of Hydrogen Halides • Two products are possible • Only 2-chloropropane is formed • H always adds to side of double bond with most hydrogens — the Markovnikov rule

  10. 57 Mechanism Acids are sources of electrophilic H+ H+ is attacked by the  electon pair ofbond leaving a carbocation a positive carbon (Double headed arrowsforelectron pairs)

  11. 57 Mechanism animation Addition of HCl to 2-methylpropene Ocol, Bcorgchm CD ROM McMurry 2.0

  12. vacant p z orbital + H 3 CH CH sp 3 3 57 Mechanism in detail • Two electrons frombond form  bond with H+,an electrophile • This leaves sp2 carbon with five electrons, a carbocation H H H H H H

  13. – Cl 3 sp 2 sp 3 sp 58 Mechanism in detail • Finally Cl– , anucleophile,donates a pair of electrons to carbocation forming a C—Cl  bond • Both carbons are sp3 hybridised. • H—Cl has added across the double bond Cl H H + H H H H CH3 H CH3 H

  14. + H + CH CHC H ( not CH CH CH ) CH CH CH 3 3 3 2 2 3 2 H CH + CH H 3 3 + + CH not C CH C C H C CH 3 2 3 2 CH CH CH 3 3 3 58 Mechanism in detail secondarycarbocation + primarycarbocation tertiarycarbocation primarycarbocation Why?

  15. CH CH CH CH CH 3 3 3 3 3 CH 3 58 Classification of carbocations • Three Groups attached to cation centre + + + C C C H H H Primary Secondary Tertiary • One Group attached to cation centre • Two Groups attached to cation centre

  16. CH CH CH CH CH 3 3 3 3 3 CH 3 + + + + + + + + C C C H H H + Primary Secondary Tertiary + + + RCH << R CH < R C 2 2 3 increasing carbocation stability 59 Stability of carbocations • Alkyl groups push electrons through sigma bonds, they are electron donating • They are positively INDUCTIVE (+I) and charge is stabilised through delocalisation or dispersion • The more alkyl groups the greater the stability • Inductive effects operate over only one to two bonds

  17. + H + CH CHC H ( not CH CH CH ) CH CH CH 3 3 3 2 2 3 2 H CH + CH H 3 3 + + CH not C CH C C H C CH 3 2 3 2 CH CH CH 3 3 3 + + + RCH << R CH < R C 2 2 3 increasing carbocation stability 59 Mechanism in detail secondarycarbocation + primarycarbocation tertiarycarbocation primarycarbocation Why?

  18. CH CH CH not formed 3 2 2 E (1°) a E (2°) a CH CH CH observed reaction 3 3 + CH CH=CH + H 3 2 59 Stability of carbocations • More stable carbocation is formed more easily • It is a lower activation energy process Energy Reaction Coordinate)

  19. CH CH CH CH CH CH 3 3 3 + 3 H – 2 3 Br C CH CH C 2 3 + CH CH CH Br 3 3 3 2-bromo-2-methylbutane 3  carbocation CH 3 not CH CH CH 3 + CH 3 2  carbocation 60 Other additions of HX An addition of HBr: C C H

  20. CH CH CH CH 3 2 3 2 – I + H CH CH CH CH CCH CH 2 3 3 2 2 3 CH CH CH CH 3 2 CH 3 2 3 3  carbocation (not 1  ) An addition of HCl: CH 3 CH CH 3 3 + Cl – + Cl H H 3  carbocation 1-Chloro-1-methylcyclohexane 61 Other additions of HX An addition of HI: I + C C 3-iodo-3-methylpentane

  21. 61 Summary • Positive part adds to the carbon of the double bond which has the greater number of hydrogens attached to it • Today this is better stated that addition of an electrophile gives the most stable carbocation • This is a general rule for addition to alkenes • All alkenes can be expected to react in this manner Markovnikov's Rule:

  22. CH 2 H X RCH CH 2 + H /H OH HO H CH 2 61 Alkene addition reactions RCH X H RCH

  23. 61 Hydration of alkenes • An example of Markovnikov addition of water

  24. 61 Hydration of alkenes Mechanism:

  25. CH 2 H X RCH CH 2 + H /H OH HO H CH 2 X 2 RCH CH 2 X X 63 Alkene addition reactions RCH X H RCH

  26. CH 2 X 2 RCH CH 2 X X 63 Addition of halogen molecules RCH

  27. + – CH CH Br + Br CH CH 2 2 2 2 Br + Br H H H but H H H H – Br then Br H 63 General mechanism Br Br X A "bridged bromonium ion" is formed Addition is trans.

  28. 63 Bromination of bacon fat Bacon fat contains unsaturated fats which add bromine Movie from Saunders General Chemistry CD-ROM

  29. 63 Addition of bromine, Br2 Addition is trans.

  30. + Br Br H H Br trans -1,2-dibromocyclohexane 63 Br2 addition to a ring • A "bridged bromonium ion" is formed • Addition gives the trans productby anti addition H H – Br

  31. – Br + Br – Br cis and trans -1,2-dibromo- cyclohexane 63 Anti-addition • A "bridged bromonium ion" explains why only trans is formed since only anti addition is possible • Stepwise addition would give the cis and trans product Br Br Br H + Br H H Br H H H H H – trans Br cis

  32. 63 Addition of chlorine • A bridged chloronium ion is formed • Cl– attacks to give the trans addition product • The reaction is general for alkenes with halogens

  33. CH 2 H X RCH CH 2 + H /H OH HO H CH 2 X 2 RCH CH 2 X X 65 Addition reactions summary RCH X H RCH

  34. cis -1,2-diol 64 Cis addition to alkenes Cis diol (glycol) formation with osmium tetroxide: • A cyclic osmic ester is first formed • Water converts this to the cisdiol

  35. cis -1,2-diol 65 Diol (glycol) formation Permanganate reacts similarly: • A cyclic manganese ester is formed • Water converts this to the cisdiol • Brown MnO2is generated (purple colour of KMnO4 lost)

  36. MnO 2 cis -1,2-cyclohexanediol 65 Diol formation — cyclic alkenes • Syn-addition to cyclic alkenes affords hydroxyl groups on the same face • Cyclic alkenes afford the cis-1,2-diol H H H H O OH O OH –

  37. CH 2 H X RCH CH 2 + H /H OH HO H CH 2 H X 2 2 RCH CH RCH CH 2 2 X X H H 65 Addition reactions summary RCH X H RCH

  38. + H2 65 Addition of hydrogen, H2 • Addition of hydrogen is exothermic by 120kJmol–1 • Addition has a very high activation energy though • With catalysts, addition occurs via a low energy path • Pt, Pd, Ni, Rh and Ru R'' R High EA R R'' R' R''' C C C C R' R''' H H

  39. 66 Catalytic action • Catalytic pathways may be multistep • Overall activation energy is lower uncatalysed high EA Energy Catalysed (multistep, low EA each step) Progress of reaction

  40. metal metal surface surface 66 Schematic of catalytic hydrogenation • Hydrogen is absorbed onto the surface • H—H bond weakened and hydrogens becomeatom-like H H H H

  41. cis - addition metal metal surface surface 66 Schematic of catalytic hydrogenation • Hydrogens react with -bond (stepwise) • Hydrogens attach on same face (syn addition) to givecis product. R R R R R R R R R R R R R R R R H H H H H H H

  42. CH 3 CH 3 H /Pt 2 CH 3 CH 3 cis -1,2-dimethylcyclohexane 67 Cis addition to cyclic alkenes • Hydrogens add to one face of the alkene • The result iscis addition H H

  43. Reaction of alkynes • Very similar to alkenes: • They add halogens twice, X2 • They add hydrogen halides twice, HX • They add hydrogen twice, H2 • They add water with acid (hydration) • Markovnikov's rule applies • They are slightly acidic and react with strong bases

  44. + Br 2 + Br Br CH CHBr2 2 2 Addition of bromine Br H • Two molecules of bromine add successively H C C H C C H Br Br H C C H Br 1,1,2,2-tetrabromoethane

  45. CH C CH CH C 3 2 CH CCH 3 3 3 Addition of HBr • Two molecules of HBr add successively • Markovnikov addition in both steps Br HBr HBr CH Br Br 2,2-dibromopropane

  46. R  R  + H 2 CH CH 2 2 Addition of H2 • Normal catalysis leads to double addition • Less active catalysts allow syn addition of one molecule R special catalyst R C C C C H H normal catalyst R' R

  47. NH NH 3 2 Reactions as an acid • Very strong base required Liq. NH3 – – R C C H R C C + Strong base Alkynide ion

  48. Alkynide formation • Anions (electron pairs) in sp hybrid orbitals arecloser to the carbon nucleus • More stable than anions in sp2or sp3orbitals

  49. 67 Index of hydrogen deficiency(Double bond equivalents) • Hexane C6H14 • Hydrogen deficiency is TWO • 2H is equivalent to either a double bond or a ring C6H12

  50. 68 Index of hydrogen deficiency • TWO double bond or • ONE ring and ONE double bond or • TWO rings Equivalentto either

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